State Highway 73 is now open after a scrub fire near Arthur's Pass closed it for much of the morning.

Crews rushed to the scrub fire on Mt Pleasant at about 2am today.

Five helicopters are battling the blaze because the area can't be reached on foot.

Fire crews thought the blaze would be catastrophic, but it's not raging as much as first thought.

Deputy Principal Rural Fire Officer Bruce James says rocks have been cleared from the road and drivers should travel with caution.

Helicopters are dropping monsoon buckets of water near sheep in harm's way to make a wall between them and the fire.